Lymphedema is a chronic debilitating disease, characterized by fibrosis, chronic inflammation, and adipose deposition in the affected extremity. In the United States and Western countries, lymphedema occurs most frequently as a complication of cancer treatment. In this setting, lymphedema occurs as a result of iatrogenic injury to the lymphatic system, most commonly after lymph node dissection, but also as a result of wide skin excisions and adjuvant therapy with radiation. Purushotham et al., J. Clin. Oncol. 23:4312-4321 (2005); Szuba et al., Cancer 95:2260-2267 (2002); Tsai et al., Ann. Surg. Oncol. 16:1959-72 (2009). Because lymph nodes are located at intersections of the limbs with the trunk, disruption of lymphatic flow results in pooling of interstitial fluid and disturbances in immune function. Baird et al., Am. J. Trop. Med. Hyg. 66:163 (2002); Sugaya et al., J. Invest. Dermatol. 132:667 (2012). It is estimated that as many as 1 in 3 patients who undergo lymph node dissection will go on to develop lymphedema, and conservative estimates suggest that as many as 50,000 new patients are diagnosed annually. DiSipio et al., Lancet Oncol. 14:500-515 (2013); Petrek et al., Cancer 83:2776-2781 (1998).
Due to the key role of surgery and adjuvant radiotherapy in the treatment of most solid tumors, lymphedema is very common, afflicting an estimated 6 million cancer survivors in the United States alone. Rockson et al., Ann. NY Acad. Sci. 1131:147 (2008); Purushotham et al., J. Clin. Oncol. 23:4312 (2005); Szuba et al., Cancer 95:2260 (2002); Tsai et al., Ann. Surg. Oncol. 16:1959 (2009). Because lymphedema is a life-long disease with no cure, the number of affected individuals is increasing annually with current estimates of over 200 million people world-wide. It is likely that this number will continue to increase in the future since the development of lymphedema is nearly linearly related with cancer survivorship, and because the prevalence of known risk factors for lymphedema, such as obesity and radiation, is rising. Erickson et al., J. Natl. Cancer Inst. 93:96-111 (2001).
Lymphedema is disfiguring and debilitating; patients have chronic swelling of the affected extremity, chronic infections, limited mobility, a decreased quality of life, and in some cases, secondary malignancies. Bicego et al., Phys. Ther. 86:1398 (2006); Hayes et al., Cancer 118:2237-2249 (2012). In addition, once lymphedema develops, it is usually progressive. Despite the fact that lymphedema is common and highly morbid, there is currently no cure, and treatment is palliative with a goal of preventing disease progression rather than restoration of lymphatic function. Velanovich et al., Am. J. Surg. 177:184-187 (1999); Beaulac et al., Arch. Surg. 137; 1253-1257 (2002). As a result, patients are required to wear tight, uncomfortable garments for the rest of their lives, in an effort to prevent lymphatic fluid buildup in the affected extremity, and to undergo intense and time consuming physical therapy treatments. Koul et al., Int. J. Radiat. Oncol. Biol. Phys., 67:841-846 (2007). In addition, despite on-going chronic care, some patients still have severe progression of their disease with increasing swelling and frequent infections in the lymphedematous limb. Currently there is no known pharmacologic therapy that can halt progression or promote resolution of lymphedema. Cormier et al., Ann. Surg. Oncol. 19:642-651 (2012). Development of targeted treatments for lymphedema is therefore an important goal and is an unmet biomedical need.
Recent studies have shown that fibrosis is not only a clinical hallmark of lymphedema, but also a key pathologic regulator of the disease. Cheville et al., Semin. Radiat. Oncol. 13:214-225 (2003); Mihara et al., PLoS One 7:e41126 (2012); Rasmussen et al., Curr. Opin. Biotechnol. 20:74-82 (2009). Inhibition of fibrotic responses preserves the capacity of the lymphatic system to transport interstitial fluid and inflammatory cells. Several lines of evidence suggest that CD4+ T cells play a central role in the pathology of lymphedema. For example, Tekola et al. recently highlighted the association between HLA class II loci and podoconiosis, a tropical form of lymphedema, and concluded that this may be a T-cell mediated inflammatory disease. Tekola et al., Trop. Med Int. Health 13:1277 (2008). Our group has previously shown that the number of CD4+ cells is increased in biopsy specimens harvested from patients with lymphedema. Avraham et al., FASEB J. 27:1114 (2013). More importantly, we have found that the number of tissue infiltrating CD4+ cells has a linear positive correlation with the severity of lymphedema in these patients. Using mouse models of lymphedema, we have shown that in contrast to wild-type (WT) mice, animals lacking T cells in general (nude mice), or CD4+ cells in particular, do not develop lymphedema following lymphatic injury. Avraham et al., FASEB J. 27:1114 (2013); Zampell et al., PloS one 7:e49940 (2012). Further, depletion of CD4+ cells with neutralizing antibodies prevents development of lymphedema. Avraham et al., FASEB J. 27:1114 (2013); Zampell et al., PloS one 7:e49940 (2012). This effect is specific to CD4+ cells since depletion of CD8+ cells or macrophages had either no effect or worsened lymphedematous skin changes. Zampell et al., PloS one 7:e49940 (2012). Finally, we have shown that Th2 differentiation of CD4+ cells is necessary for development of pathologic changes of lymphedema including fibrosis, impaired lymphangiogenesis, and impaired collecting lymphatic function. Avraham et al., FASEB J. 27:1114 (2013); Savetsky et al., PloS one 10:e012908 (2015).
However, while it is clear that CD4+ cells play a key role in the pathology of lymphedema, the mechanisms that regulate CD4+ activation, differentiation, and homing to lymphedematous tissues remains unknown. For example, it is unclear if naïve CD4+ cells require activation in skin draining lymph nodes in order to differentiate into effector cells that can infiltrate lymphedematous tissues. Additionally, the cellular signals that guide homing of effector CD4+ cells to lymphedematous tissues remain unknown. This gap in our knowledge is important, since identifying the mechanisms that regulate CD4+ cell activation and homing after lymphatic injury may identify novel treatment options for this disabling disease.
There are currently no pharmacologic therapies available for the treatment of lymphedema. Coumarin has been used in patients with lymphedema with modest success. Casley-Smith et al., BMJ 307:1037-1041 (1993); Casley-Smith et al., N. Engl. J. Med. 329:1158-1163 (1993); Casley-Smith et al., Australas J. Dermatol. 33:69-74 (1992); Loprinzi et al., N. Engl. J. Med. 340:346-350 (1999). However, widespread clinical application of this drug has been hampered by significant toxicity including liver failure and death. Loprinzi et al., N. Engl. J. Med. 340:346-350 (1999). Although highly effective, systemic depletion of CD4+ cells is not clinically viable due to unacceptable morbidity and systemic complications such as infections, cancer recurrence, and autoimmune disorders. Accordingly, there is a need in the art for novel treatments for lymphedema.
FTY720 is a small molecule agonist of the sphingosine 1-phosphate (S1P) receptor. It is derived from the antibiotic myriocin, and is a structural analogue of sphingosine. When phosphorylated by sphingosine kinase, FTY720 causes the internalization of S1P receptors, which blocks lymphocyte egress from the lymph nodes. Mandala et al., Science 296:346-349 (2002). FTY720 is FDA-approved under the brand name Gilenya® (fingolimod) for the treatment of patients with relapsing forms of multiple sclerosis (MS). We show here for the first time that blockade of T cell release from lymph nodes by downregulation of S1P using FTY720 prevented accumulation of activated T cells in skin, and potently inhibited development of lymphedema, showing that this approach can have clinical utility.